It's very important to seal off all the "bypass" openings in the
attic. That means all the plumbing that comes through the ceiling, the
kitchen stove vent hood, the furnace vent, etc.

For the furnace vent, pick up some aluminum sheeting at the home
center and cut it around the furnace vent. Seal the it to the actual
vent using intumescent sealer (pick it up in the caulk section). Use
intumescent foam to seal all the plumbing openings and around each
electrical box the punctures the ceiling.

Intumescent foam will not burn up in a fire. It actually expands when
heat is applied. Code requires that type of foam and caulk.